A system used within mixing consoles to allow specific channel signals to be monitored in isolation. Solo differs from PFL and AFL in that it doesn't route signals to the monitoring system via a dedicated bus. Instead, it uses the console's main stereo output bus and works by muting all other channels, leaving only the channel currently being solo'd to feed the stereo mix bus. For this reason, it is often called a 'destructive solo' and the function is normally locked-out in broadcast desks when the 'red light' is turned on! As the Solo'd signal includes the effects of both the fader and the pan-pot, it is sometimes known as 'solo-in-place, or SIP. (See AFL and PFL).
Most consoles have a 'Solo Safe' function to prevent certain channels from being muted when others are solo'd. This would typically be used to maintain the signals from effects returns, for example, so that a solo'd channel can be auditioned with its related effects.